feat: implement validator passphrase

[shahin-hq]

Summary

Closes: https://app.clickup.com/t/2570579/86e1kefb0

This PR refactors validator registration/update to use passphrase instead of public key.

I used the following snippet for testing with SDK tester:

builder = newValidatorRegistrationBuilder()
.validatorPassphrase((String) validatorConfig.get("validatorPassphrase"))
.gasPrice(parseLong(validatorConfig.get("gasPrice")))
.gasLimit(parseLong(validatorConfig.get("gasLimit")))
.nonce(nonce.longValue())
.value((newBigInteger("250").multiply(BigInteger.TEN.pow(18))).toString())
.recipientAddress((String) wellKnownContracts.get("consensus"));

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged